Pivotal bone anchor assembly with multi-part shank retainer and rod-engaging insert

Abstract

A polyaxial bone screw assembly includes a threaded shank body having an upper capture structure, a head and a multi-piece retainer, articulation structure. The geometry of the retainer structure pieces corresponds and cooperates with the external geometry of the capture structure to frictionally envelope the retainer structure between the capture structure and an internal surface defining a cavity of the head. The head has a U-shaped cradle defining a channel for receiving a spinal fixation or stabilization longitudinal connecting member. The head channel communicates with the cavity and further with a restrictive opening that receives retainer pieces and the capture structure into the head but prevents passage of frictionally engaged retainer and capture structures out of the head. The retainer structure includes a substantially spherical surface that mates with the internal surface of the head, providing a ball joint, enabling the head to be disposed at an angle relative to the shank body. Methods of installation are also provided.

What is claimed is: 
     
       1. A pivotal bone anchor assembly for securing a rod to a bone of a patient, the pivotal bone anchor assembly comprising:
 a shank having an elongate body with a capture structure at a distal end having a spherical shape, the elongate body configured for fixation to the bone; 
 a receiver having a top portion and a base, the top portion configured to receive the rod, the base having a non-cylindrical internal locking surface at least partially defining a cavity, the receiver having a central bore communicating with the cavity, the cavity communicating with an exterior of the base through a lower opening in the base, the lower opening sized and shaped to receive the capture structure of the shank therethrough; 
 a retainer structure having at least two discrete retainer parts, each retainer part being positionable within the cavity and having an outer surface and a curved concave inner surface, the retainer parts configured to at least partially surround and capture the capture structure between the curved concave inner surfaces, with the outer surfaces of the retainer parts being frictionally engageable with the internal locking surface of the base, the shank being pivotal with respect to the receiver when the retainer structure is positioned between the capture structure and the internal locking surface during assembly of the pivotal bone anchor assembly and prior to the rod being locked in place within the top portion of the receiver, the retainer structure being configured to at least partially inhibit the shank from moving up out of the retainer structure after the capture structure is captured therein; and 
 a rod-engaging insert positionable in the central bore of the receiver between the rod and the capture structure. 
 
     
     
       2. The pivotal bone anchor assembly of  claim 1 , wherein each retainer part is configured for positioning in the cavity prior to the capture structure. 
     
     
       3. The pivotal bone anchor assembly of  claim 1 , wherein the rod-engaging insert is engageable with each of the retainer parts. 
     
     
       4. The pivotal bone anchor assembly of  claim 1 , wherein the outer surfaces of the retainer parts and the internal locking surface of the base are complementary. 
     
     
       5. The pivotal bone anchor assembly of  claim 1 , wherein the outer surfaces of the retainer parts and the internal locking surface of the base are at least partially spherical. 
     
     
       6. The pivotal bone anchor assembly of  claim 1 , wherein the curved concave inner surfaces of the retainer parts frictionally engage the capture structure. 
     
     
       7. The pivotal bone anchor assembly of  claim 1 , wherein the at least two discrete retainer parts further comprises a plurality of parts. 
     
     
       8. The pivotal bone anchor assembly of  claim 1 , wherein the at least two discrete retainer parts further comprises first and second retainer parts. 
     
     
       9. The pivotal bone anchor assembly of  claim 8 , wherein the second retainer part is a substantial mirror image of the first retainer part. 
     
     
       10. The pivotal bone anchor assembly of  claim 8 , wherein the first and second retainer parts have different shapes. 
     
     
       11. The pivotal bone anchor assembly of  claim 8 , wherein the first and second retainer parts are completely separate and spaced-apart from each other in a gapped arrangement when the rod is locked in place within the top portion of the receiver. 
     
     
       12. The pivotal bone anchor assembly of  claim 1 , wherein the capture structure of the shank has a tool engagement formation disposed thereon adapted for non-slip tool engagement to drive the elongate body of the shank into the bone. 
     
     
       13. The pivotal bone anchor assembly of  claim 1 , wherein the capture structure of the shank includes at least one inward groove or outward ridge that cooperates with the retainer structure. 
     
     
       14. The pivotal bone anchor assembly of  claim 1 , wherein the rod-engaging insert is top loaded into the central bore of the receiver. 
     
     
       15. The pivotal bone anchor assembly of  claim 1 , wherein the shank includes a central opening along an entire length thereof. 
     
     
       16. The pivotal bone anchor assembly of  claim 1 , wherein the shank includes a top end surface extending above a top surface on the retainer structure when the capture structure of the shank is captured by the retainer structure in the cavity, the top end surface configured to engage the insert positioned in the central bore of the receiver. 
     
     
       17. The pivotal bone anchor assembly of  claim 1 , wherein each retainer part is sized and shaped to be loadable into the receiver through at least one of an open channel formed in the top portion of the receiver or the lower opening in the base, and wherein the capture structure of the shank is sized and shaped to be loadable into the receiver through the lower opening in the base. 
     
     
       18. The pivotal bone anchor assembly of  claim 1 , further comprising a closure structure insertable into the receiver, and wherein downward displacement of the closure structure within the receiver results in the outer surface of each retainer part being urged against the internal locking surface of the base, thereby locking the elongate body of the shank in a selected angle with respect to the receiver. 
     
     
       19. The pivotal bone anchor assembly of  claim 18 , wherein the receiver has upstanding spaced-apart arms defining an open channel, the arms having guide and advancement structures on an inside surface thereof, and the closure structure is configured for downward displacement within the receiver via rotation of the closure structure along the guide and advancement structures. 
     
     
       20. The pivotal bone anchor assembly of  claim 1 , wherein the cavity includes a width which decreases toward the lower opening in the base. 
     
     
       21. A pivotal bone anchor assembly for securing a rod to a bone of a patient, the pivotal bone anchor assembly comprising:
 a shank having an elongate body, a driving tip and a capture structure, the elongate body disposed between the driving tip and the capture structure, the elongate body configured for fixation to the bone, the capture structure having a spherical upper region and a spherical lower region opposite the spherical upper region, the elongate body extending downwardly towards the driving tip from the spherical lower region; 
 a receiver having a top portion and a base, the top portion defining an open channel configured to receive the rod, the base having an interior surface at least partially defining a cavity of the base, the open channel communicating with the cavity, the cavity communicating with an exterior of the base through a bottom opening in the base, the bottom opening being sized and shaped to receive the capture structure of the shank therethrough; and 
 a retainer structure having first and second retainer pieces, the first retainer piece having a first inner surface and a first outer surface, the second retainer piece having a second inner surface and a second outer surface, the first and second inner surfaces configured for frictional engagement with the capture structure and the first and second outer surfaces configured for frictional engagement with the interior surface that at least partially defines the cavity of the base, the first and second retainer pieces configured to hold the capture structure in the cavity of during angular positioning of the shank relative to the receiver, 
 wherein the first and second retainer pieces are positionable within the cavity prior to the capture structure. 
 
     
     
       22. The pivotal bone anchor assembly of  claim 21 , further comprising an insert positionable in the receiver between the capture structure of the shank and the rod positionable in the open channel. 
     
     
       23. The pivotal bone anchor assembly of  claim 21 , wherein the first and second retainer pieces have an overlapping relationship with respect to each other. 
     
     
       24. The pivotal bone anchor assembly of  claim 21 , wherein the first retainer piece includes a recessed portion and the second retainer piece has a projecting portion, the first and second retainer pieces having an overlapping configuration when positioned in the cavity of the base. 
     
     
       25. The pivotal bone anchor assembly of  claim 21 ,
 wherein the first and second retainer pieces are sized and shaped to be loadable into the receiver through one of the open channel and the bottom opening, and 
 wherein the capture structure of the shank is sized and shaped to be loadable into the receiver through the bottom opening in the base. 
 
     
     
       26. The pivotal bone anchor assembly of  claim 21 , wherein the first and second inner surfaces of the first and second retainer pieces generally three dimensionally match an outer surface of the capture structure of the shank so as to matingly receive and contact the outer surface of the capture structure between the first and second retainer pieces. 
     
     
       27. A pivotal bone anchor assembly for securing a rod to a bone of a patient, the pivotal bone anchor assembly comprising:
 a shank having an elongate body and a capture structure, the elongate body being configured for fixation to the bone; 
 a receiver having a top portion and a base, the receiver top portion defining a channel to receive the rod, the base having a non-cylindrical internal surface partially defining a cavity, the channel communicating with the cavity, the cavity communicating with an exterior of the base through an opening configured to upward load the capture structure of the shank through the opening; and 
 a retainer structure having at least two discrete retainer parts, each retainer part having an inner surface and an outer surface, the inner surface configured to be in frictional engagement with the capture structure and the outer surface configured to be in frictional engagement with the non- cylindrical internal surface when the capture structure is captured by the retainer structure, the retainer parts cooperating to prevent the shank capture structure from exiting the receiver cavity through the opening, 
 wherein the retainer structure is configured for pivotal movement with respect to the receiver prior to the pivotal bone anchor assembly being locked by a closure. 
 
     
     
       28. The pivotal bone anchor assembly of  claim 27 , further comprising an insert positionable in the receiver between the retainer structure and the rod. 
     
     
       29. The pivotal bone anchor assembly of  claim 28 , wherein the insert is configured to set the position of the shank with respect to the receiver prior to insertion of the rod into the receiver channel. 
     
     
       30. The pivotal bone anchor assembly of  claim 28 , wherein the insert is inhibited from moving upward within the receiver after being positioned therein. 
     
     
       31. A pivotal bone fixation assembly for securing a fixation rod to a bone of a patient, the pivotal bone fixation assembly comprising:
 a bone fastener having a head and a bone-engaging portion; 
 a receiver defining a central through space along a first axis for receiving the bone fastener at a variable angle, the receiver having a non-cylindrical inner surface in a lower portion of the receiver adjacent a bottom opening of the through space; and 
 a retainer with at least two discrete retainer parts positionable in the receiver lower portion, each retainer part having a planar top surface perpendicular to a second axis of the retainer and an inclined surface with respect to the planar top surface so as to provide clearance space for the at least two retainer parts to pivot relative to each other when installing the head of the bone fastener into the retainer within the receiver through the bottom opening of the receiver through space. 
 
     
     
       32. The pivotal bone fixation assembly of  claim 31 , wherein at least one of the two retainer parts includes a recess or notch formed in a surface thereof. 
     
     
       33. The pivotal bone fixation assembly of  claim 31 , wherein the bottom opening is circular in shape for receiving the head of the bone fastener therethrough. 
     
     
       34. The pivotal bone fixation assembly of  claim 31 , wherein the at least two retainer parts are configured to move apart with respect to each other when in the receiver so as to capture the head of the bone fastener therein. 
     
     
       35. The pivotal bone fixation assembly of  claim 31 , wherein the at least two retainer parts are sized and shaped for allowing variable angulation of the bone fastener with respect to the receiver. 
     
     
       36. The pivotal bone fixation assembly of  claim 35 , wherein the at least two retainer parts are further sized and shaped for preventing removal of the bone fastener from the retainer and the receiver after being captured therein. 
     
     
       37. The pivotal bone fixation assembly of  claim 31 , wherein the receiver defines an open channel extending along an axis generally perpendicular to the first axis. 
     
     
       38. The pivotal bone fixation assembly of  claim 37 , wherein the channel is sized and shaped for receiving the fixation rod. 
     
     
       39. The pivotal bone fixation assembly of  claim 31 , wherein the at least two retainer parts include a first retainer part and a second retainer part. 
     
     
       40. The pivotal bone fixation assembly of  claim 39 , wherein the first retainer part is configured for mating attachment to the second retainer part about the second axis of the retainer. 
     
     
       41. The pivotal bone fixation assembly of  claim 39 , wherein the first retainer part cooperates with the second retainer part to surround an outer perimeter of the bone fastener head. 
     
     
       42. A pivotal bone fixation assembly for securing a rod to a bone of a patient, the pivotal bone fixation assembly comprising:
 a bone fastener having a head and a bone-engaging portion; 
 a receiver defining an opening along a first axis for receiving the bone fastener at a variable angle, the receiver having an inner articulation surface at a lower portion of the opening; and 
 first and second retainer parts configured to be coupled to the bone fastener head for movement therewith, the first and second retainer parts bcing configured to be matingly articulable with the inner articulation surface of the receiver, with at least one retainer part including a notch for receiving a portion of the other retainer part during assembly of the first and second parts with the bone fastener head in the receiver. 
 
     
     
       43. The pivotal bone fixation assembly of  claim 42 , wherein the first and second retainer parts are configured to be at least partially overlapping during assembly. 
     
     
       44. The pivotal bone fixation assembly of  claim 42 , wherein the first retainer part and the second retainer part are configured to cooperate to provide a discontinuous annular curved surface with a three-hundred-and-sixty degree (360°) circumference surrounding the head of the bone fastener so as to capture the head between the retainer parts.